Disk drives are an important data storage technology, which is based on several crucial components. These components include the interconnection between the read/write heads, which actually communicate with a disk surface containing the data storage medium, and the read/write interfaces of the disk drive. This invention involves the control of the read/write heads during read operations in terms of setting at least the read mechanism's current.
FIG. 1A illustrates a typical prior art high capacity disk drive 10 including actuator arm 30 with voice coil 32, actuator axis 40, suspension or head arms 50-58 with slider/head unit 60 placed among the disks.
FIG. 1B illustrates a typical prior art high capacity disk drive 10 with actuator 20 including actuator arm 30 with voice coil 32, actuator axis 40, head arms 50-56 and slider/head units 60-66 with the disks removed.
Since the 1980's, high capacity disk drives 10 have used voice coil actuators 20-66 to position their read/write heads over specific tracks. The heads are mounted on head sliders 60-66, which float a small distance off the disk drive surface when in operation. Often there is one head per head slider for a given disk drive surface. There are usually multiple heads in a single disk drive, but for economic reasons, usually only one voice coil actuator.
Voice coil actuators are further composed of a fixed magnet actuator 20 interacting with a time varying electromagnetic field induced by voice coil 32 to provide a lever action via actuator axis 40. The lever action acts to move head arms 50-56 positioning head slider units 60-66 over specific tracks with speed and accuracy. Actuator arms 30 are often considered to include voice coil 32, actuator axis 40, head arms 50-56 and head sliders 60-66. Note that actuator arms 30 may have as few as a single head arm 50. Note also that a single head arm 52 may connect with two head sliders 62 and 64.
Merged type heads possess different components for reading and writing, because the magneto-resistive effect only occurs during reading. A merged type head typically includes a thin film head and a spin valve sensor. The primary use of the thin film head is in the write process. The spin valve sensor is used for reading.
MR heads have several advantages over earlier approaches, using a single component, for both read and write. Earlier read/write heads were a study in tradeoffs. The single component, often a ferrite core, would have increased read sensitivity with additional windings about the core. However, these added windings made the ferrite core write less efficiently.
Introduced in the 1990's, merged heads brought significant increases in areal density. A merged type head reads the disk surface using a conductive thin film, the spin valve, whose resistance changes in the presence of a magnetic field. By separating the functions of writing and reading, each function can be optimized further than would be possible for the older read/write heads. For all the improvement that merged heads bring, their control to date creates problems. However, before discussing these problems, consider first how and what controls these devices in contemporary disk drives.
FIG. 2A illustrates a simplified schematic of a disk drive controller 1000 controlling an analog read/write interface 220, the read differential signal pair (r+ and r−) and write differential signal pair (w+ and w−) communicating the resistivity found in the spin valve within MR read/write head 200 of the prior art.
FIG. 2B illustrates Max_Ir table 1124 as found in FIG. 2A.
Computer 1100 within embedded disk controller 1000 receives readings of the spin valve resistance R_rd from analog read/write interface 220. Computer 1100 also controls the read current Ir_set for read differential signal pair r+ and r−, as well as the write current Iw_set for write differential signal pair w+ and w−.
The process of reading the data storage surface using MR read/write head 200 includes the following. Computer 1100 accesses 1122 a memory 1120. Memory 1120 contains program system 1128 and the Max Ir table 1124. Program system 1128 and Max Ir table 1124 are part of the process of determining R_rd, Ir_set, and Iw_set, as well as asserting currents on the read and write differential signal pairs.
Memory 1120 typically includes a non-volatile memory component. This non-volatile memory component is often used to store program system 1128 as well as Max Ir table 1124.
Today, a disk drive performs an initialization process often known as read channel optimization. Read channel optimization is supposed to find the best parameters for read/write operation, which include at least read a bias current (Ir), write current Iw and write boost. To prevent damage and/or degradation of the read head due to high Ir during and after read channel optimization, a maximum allowed Ir 1160 based upon a measured hot MR read resistance 1170 is placed the Max Ir table 1124.
Generating the table includes measuring the hot MR read resistance typically at a value such as Ir=4 mA, and then selecting a read bias current Ir lower than the maximum allowed Ir current Ir 1160 associated with the measured hot MR read resistance 1170 as found in the table 1124.
The Max Ir table 1124 is usually based on the lifetime estimation of the spin valve and can be determined at a head arm component level after a stress test often known as an “electro-migration test”. The test conditions are typically over a range such as Ir from 4 mA to 7 mA and temperature from 110 degrees C. to 150 degrees C. Such tables often have the form shown in Table 1.
TABLE 1A look-up table 1128 determining the maximum readcurrent Ir based upon read bias resistance.IrMax Hot MR ReadRead CurrentResistance4.0 mA55 ohm4.4 mA48 ohm4.8 mA44 ohm5.2 mA41 ohm5.6 mA38 ohm6.0 mA36 ohm6.4 mA33 ohm6.8 mA31 ohm
FIG. 3A illustrates a detail flowchart of prior art program system 1128 of FIG. 2A for operating MR read/write head 200.
Arrow 1200 directs the flow of execution from starting operation 1128 to operation 1202. Operation 1202 performs read channel optimizing MR read/write head 200. Arrow 1204 directs execution from operation 1202 to operation 1206. Operation 1206 terminates the operations of this flowchart.
Arrow 1210 directs the flow of execution from starting operation 1128 to operation 1212. Operation 1212 performs determining the maximum read bias current Ir based upon measured hot resistance for read channel. Arrow 1214 directs execution from operation 1212 to operation 1206. Operation 1206 terminates the operations of this flowchart.
FIG. 3B illustrates a detail flowchart of prior art operation 1212 of FIG. 3A for determining the maximum read bias current Ir based upon measured hot resistance for read channel.
Arrow 1230 directs the flow of execution from starting operation 1212 to operation 1232. Operation 1232 performs measuring the hot resistance of the read channel of MR read/write head 200. Arrow 1234 directs execution from operation 1232 to operation 1236. Operation 1236 terminates the operations of this flowchart.
Arrow 1240 directs the flow of execution from starting operation 1212 to operation 1242. Operation 1242 performs determining maximum read bias current Ir based upon measured hot resistance of read channel using Max Ir table 1124. Arrow 1244 directs execution from operation 1242 to operation 1236. Operation 1236 terminates the operations of this flowchart.
While this has been the accepted way to control the read bias current for many years, it does not fully account for the physical situation in which these merged read/write head are used.
The temperature of a spin-valve depends upon the inside temperature of the disk drive, read bias current Ir, MR resistance of the spin valve and interconnect design between the analog interface, particularly the preamplifier and the head slider.
However, the writing operation is not considered in the prior art. When either the operational frequency rises and/or the total number of read/write heads within a disk drive decrease, the read mechanism experiences a significant temperature rise due to writing. This is particularly true when both the operational frequency rises and the total number of read/write heads within the disk drive decrease. Note that even a 10 degree, much less 40 degree, Centigrade temperature rise may significantly effect read mechanism stability and/or overall disk drive life expectancy.
The nature of the significance is two-fold: Increased temperatures tend to make the read mechanism less stable. The lifetime of the merged read/write head may be shortened.
FIG. 4A presents findings by the inventors showing the MR resistance of a spin valve in Ohms along the vertical axis and various combinations of write currents Iw and operational frequencies for two read bias Ir current settings of 1 mA and 4.5 mA.
FIG. 4B presents findings by the inventors showing the temperature of a spin valve in degrees Centigrade along the vertical axis and various combinations of write currents Iw and operational frequencies for two read bias Ir current settings of 1 mA and 4.5 mA.
Note that as the write current and operational frequency increase, the hot read resistance and head temperature rise. Additionally, both the hot read resistance and head temperature can be further seen to rise with increase in read bias current Ir, for any given combination of Iw and operational frequency.
FIG. 4B further shows over a 90 degree centigrade temperature increase in the read mechanism at Iw=40 mA, operating frequency=260 MHz and Ir=4.5 mA, which is over 50 degrees greater than the situation when Iw=0 mA and operating frequency is not taken into account. Such conditions indicate a significant heating of the read mechanism by the write mechanism, tending to reduce the stability of the read mechanism and the life expectancy of the disk drive where this occurs.